Technical Field
This disclosure generally relates to nanostructure-based transparent conductors having increased haze or light-scattering.
Description of the Related Art
Transparent conductors are optically clear and electrically conductive films. They are in widespread use in areas of display, touch-panel, photovoltaic (PV), various types of e-paper, electrostatic shielding, heating or anti-reflective coatings (e.g., windows), etc. Various technologies have produced transparent conductors based on one or more conductive media such as metallic nanostructures, transparent conductive oxides (TCO), conductive polymers, and/or carbon nanotubes.
Depending on the end use, transparent conductors may be created to satisfy certain electrical and optical parameters, including, for example, sheet resistance, optical transparency, and haze. Often, production of transparent conductors requires balancing trade-offs between the electrical and optical performances.
While displays and touch panels generally require that transparent conductors have low haze, high light transmission, and low sheet resistance, there are applications in which high or increased haze is to be combined with low sheet resistance, high light transmission, low reflectivity and low absorption. A predominant application is thin-film PV whereby an increased haze in the transparent electrode assists to increase the coupling of external light into a thin-film PV stack via increased scattering of the incoming light into the semiconducting PV stack. Increased haze or light-scattering is currently achieved through surface-textured TCOs, which are commonly prepared by chemical treatment (e.g., wet-chemical etch) of a pre-deposited TCO (of lower haze) or depositing a high-haze TCO (e.g., a sol gel approach). However, this approach is limited by the nature of the TCOs (e.g., their chemical and physical micro-structure) in the types of scattering structure that can be achieved.
Metallic nanostructure-based transparent conductors may display haze/light-scattering due to the presence of the nanostructures (e.g., nanowires). As a general rule for nanostructure-based transparent conductors, lower sheet resistance is typically associated with more conductive nanostructures, which in turn results in lower light transmission and higher haze. Nevertheless, in a typical nanostructure-based transparent conductor at a sheet resistance in the 10 Ohms/square (a typical value for PV applications), the haze (photoptically measured) is in the 5%-8% range. Although this level of light-scattering is typically too high for displays, touch-panel and certain other applications, it may be too low for thin film PV applications. In particular, for silicon-based thin film PV such as a-Si or um-Si or a-Si/um-Si tandem PV cells and modules, a haze value of at least 15% is typically targeted. Thin-film photovoltaic devices that employ other photovoltaic elements such as organic PV, organic-inorganic hybrid PV, copper indium gallium (di)selenide (CIGS) and CdTe may also benefit from relatively high haze transparent conductors.